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A method of operation of a memory array for storage of records of differing
predetermined sizes is disclosed which features division of the array into
domains which are substantially integral multiples of the predetermined
record sizes. In a preferred embodiment the domain allocation may be
varied adaptively in accordance with usage so as to enable more efficient
use of the array.

This application is a continuation-in-part of copending Ser. No. 325,351,
filed Nov. 27, 1981, now U.S. Pat. No. 4,430,712 issued Feb. 7, 1984,
having the same title.

Claims

We claim:

1. In a method of dividing a memory array into domains for the storage of data received in plural differing predetermined amounts from plural classes of storage devices, the improvement
which comprises:

effectively subdividing said array into domains of sizes which are each substantially integral multiples of said predetermined amounts of data stored therein by assigning starting addresses to said domains.

2. The improvement of claim 1 wherein said domains are allocated to one class of device at an initialization stage.

3. The improvement of claim 2 wherein the allocation of said domains may be varied from device class to device class over time, said variation being controlled in accordance with the ratio of the number of domains allocated to a particular
device type to usage of said domains, and being accomplished by subdividing domains to be reallocated into areas of size equal to one of said predetermined amounts of data.

4. The improvement of claim 1 wherein said predetermined amounts of data are equivalent to complete tracks of data stored on magnetic disk storage media.

5. A method of organizing an array of memory locations to be used to store data records of plural differing predetermined lengths comprising the steps of:

determining a domain size which is substantially an integral multiple of all said plural predetermined data amounts;

effectively dividing said array into domains of the thus determined domain size by assigning starting addresses in said array to each domain; and

allocating each of said domains to storage of records of a single one of said predetermined data amounts.

6. The method of claim 5 wherein said allocation may be varied in accordance with usage of each of said domains.

7. A caching memory subsystem for attachment to a host computer and magnetic disk devices of differing classes, in which various predetermined amounts of data are stored on the tracks of the disk of each class of disk devices comprising:

a solid-state cache memory array and a cache manager, wherein said solid-state cache memory array is effectively divided into plural domains by assignment of starting addresses in said array to each domain, said domains being so sized as to
contain substantially integral multiples of said predetermined amounts of data;

wherein said cache manager has means to divide each of said domains into frames sized to contain an amount of data equal to one of said predetermined amounts of data, and means to assign the address of a frame of corresponding size to data to be
copied from one of said disk tracks to said solid-state caching memory array.

8. The caching memory subsystem of claim 7 wherein said cache manager further comprises means for reassignment of particular ones of said domains from one class of disk device to a second class upon indication that a usage imbalance exists, and
to thereupon redivide said reassigned domains into frames sized to store amounts of data corresponding to the predetermined amount of data stored on one track of said disk device of the second class.

Description

FIELD OF THE INVENTION

This invention relates to data storage management and control. More particularly, the invention relates to a system wherein a solid-state cache memory used in conjunction with long-term magnetic disk storage devices of varying types is
subdivided into blocks or "domains" of sizes chosen so as to ensure efficient cache memory utilization despite varying demands placed on the system.

BACKGROUND OF THE INVENTION

The present invention relates to a solid-state cache memory subsystem designed to be used as an adjunct to long-term magnetic disk storage media. The cache memory is connected outboard of a conventional data transmission channel not specifically
designed for use with such a cache memory subsystem, such that the cache memory system is plug-compatible with and software transparent to a host computer. In particular, the invention is designed to operate in accordance with the cache memory subsystem
which is the subject matter of copending Ser. No. 325,346 filed Nov. 27, 1981, Dodd, "Cache Buffered Memory Subsystem", incorporated herein by reference. That application describes a cache memory subsystem which is operatively connected to storage
director(s) which, in turn, are connected between a host computer at one terminus and the control modules at another. In turn the control modules connect to the actual disk drive. As is known in the art, the directors serve to interface the host with
the control modules which, in turn, interface the directors to the disk drives. The control modules are selected in accordance with a particular type of disk drive used, while the directors may be used with various types of control modules and hence
various types of disk drives. According to the invention of that application, the cache is adapted to be used with varying sorts of disk drives and control modules by virtue of direct connection to the director(s).

The function of the solid-state cache memory subsystem in the invention of the copending application referred to above is to store certain data which has been determined to be likely to be called for by the host in advance of an actual host
request for the data. This determination is done in accordance with the invention disclosed in further copending applications, Ser. No. 325,350, filed Nov. 27, 1981, Coulson et al, "Detection of Sequential Data Streams", and in a continuation-in-part
of that application, Ser. No. 441,901 filed Nov. 15, 1982, having the same title, both incorporated herein by reference. In a preferred embodiment of that invention, once it has been determined that a particular block of data is likely to be called
for by the host, the entire disk "track" containing that block of data is brought into the cache memory for temporary storage in anticipation of its being called for by the host computer.

It is desirable for reasons of addressing convenience to subdivide the cache memory into "frames" which are sized to store blocks of data of the sizes which can be expected to be stored therein. Inasmuch as the cache memory subsystem must be
adapted to cooperate with varying types of disk drives which in turn have varying amounts of data stored on a given track, means must be provided whereby the cache memory is as useful as is possible despite the fact that disk tracks of varying sizes will
be stored therein track by track. For example, if it were assumed that the total cache memory was to contain 200 kilobytes (hereinafter 200 kb) of data, and the track length of a particular disk drive was 20 kb, 10 such tracks would fit into the cache.
Stated slightly differently, the cache could be divided into ten 20-kb "frames." When, for example, all ten frames were full and it was desired to cache another track, a cache manager could then simply deallocate a frame, for example, the frame
containing the data which had been least recently used and then allocate that frame to the new track. However, if data stored on another type of disk drive having a track size of, say, 25 kg, were desired to be staged, it would be necessary to
deallocate two adjacent 20 kb frames in order to accommodate a 25 kb track thus wasting 15 kb of space. Even assuming two adjacent frames could be deallocated without loss of useful data, the waste of space alone would clearly be undesirable.

If, on the other hand, the entire solid-state memory were divided into two subportions or "domains" one adapted to store only tracks of one size, and another proportioned to store tracks of another size, the individual domains of the memory might
be efficiently subdivided for allocation to tracks of a particular size. However, the utility of this solution presupposes that the relative activity of the disks having the two sizes of tracks is constant over time, which cannot be expected to be the
case. Thus, it is desirable that all area of the solid-state memory be accessible to any track size supported by the cache in order that it can be most efficiently used. Finally, one could divide the cache memory into much smaller, uniformly-sized
"pages", e.g., 2 kb, into which a track of any size could be divided. However, this would require complex hardware to implement and would additionally reduce the efficiency of the cache as an individual disk track would necessarily tend to be stored on
varying portions of the cache, which would entail substantial additional support, e.g., recall of the numerous storage locations of the portions of a given track.

OBJECTS OF THE INVENTION

It is accordingly an object of the present invention to provide a means whereby the storage area available in a solid-state cache memory can be most efficiently used.

It is a further object of the invention to provide a means whereby a solid-state cache memory subsystem can be operated efficiently to store data records of varying sizes.

It is a further object of the invention to provide a means for "adaptively" or "dynamically" reconfiguring a cache memory previously divided into domains of plural classes in accordance with actual use, so as to fully utilize the memory space
available.

SUMMARY OF THE INVENTION

The above needs of the art and objects of the invention are satisfied by the present invention according to which a solid-state memory is subdivided into domains. The size of the domains is chosen to be a substantially integral multiple of each
of the track sizes desired to be stored therein, such that, for example, three 50-kb tracks from one class of device will fit within the same 150-kb domain as two 75-kb tracks from another class. Each individual domain is thereafter assigned to storage
of tracks from a given type of device, and then divided into track-sized frames, to which individual tracks are written. Statistics are maintained on domain usage. If the indications are that the data staged from a particular device type predominates
over another, a given domain is reallocated from the second device type to the first device type so as to insure efficient usage of the total memory space available. In this way, fragmentation due to intermingling of varying track sizes in the same area
of memory is avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood if reference is made to the accompanying drawings, in which:

FIG. 1 represents a schematic block diagram view of the system of the invention;

FIG. 2 shows how individual domains can be subdivided according to the track sizes desired to be stored therein;

FIG. 3 shows a domain switching decision table used in determining which domains to switch at a particular time;

FIG. 4 shows a diagram indicating how the domain switching determination decision is utilized in actually performing the switching operation and how the control tables are updated as the scheme proceeds; and

FIGS. 5a-5c depict the least recently used (LRU) list used in assigning frames to data to be stored.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As discussed above, the adaptive domain partitioning methods of the invention are designed to be used in a solid-state cache memory subsystem operating to contain data being stored on disk memory in anticipation of its being called for by a host
computer.

FIG. 1 shows a schematic diagram of such a system. The host computer 10 is connected by means of a conventional channel interface 12 to a storage director 16. The storage director in turn interfaces to plural control modules 18 which themselves
control disk drives 14 which as shown may be of differing types. In particular, the amount of data stored on a given track of one of the disks 14 can vary. When a decision is made by a cache manager 24, e.g., in accordance with the copending
applications referred to above having the title "Detection of Sequential Data Stream", that it would be desirable to store the contents of a track in a solid-state cache memory array 22, a number of locations in the cache corresponding in size to the
length of the track on the disk 14--i.e., a "frame" of locations in the cache--must be assigned to the data. Accordingly when it is desired by the cache manager 24 that a particular track be staged into a frame in the cache array 22, a frame of
appropriate size is assigned to that track and the operation proceeds.

Those skilled in the art will recognize that such "framing" of a block of contiguous memory locations and assignment thereof is desirable to reduce the complexity of assigning addresses to the data. In particular, if this is done, the
beginning-of-track (BOT) and the end-of-track (EOT) addresses only need to be known to fully define the locations of all the data on the track, as the total number of data bytes is defined by the frame definition. See copending Ser. No. 440,334 filed
Nov. 9, 1982, Swiatek et al, "Dynamic Addressing for Variable Track Length Cache Memory", and Ser. No. 325,288, filed Nov. 27, 1981, Moreno et al for "Control of Cache Buffer for Memory Subsystem", and a continuation-in-part of that application Ser.
No. 441,818 filed Nov. 15, 1982, having the same title, all of which are incorporated herein by reference, for details of assignment of cache memory addresses to data to be stored in the frames. The present invention is concerned with efficient
division of the cache array into subportions or "domains." The size of the domains is chosen so that approximately integral numbers of disk tracks of varying lengths fit into "frames" in each domain. Thus, after allocation of the domain to a particular
track length, it is divided into "frames" of sizes corresponding to the track lengths.

Data records from a track on a magnetic disk are stored in a frame in a cache memory which is described by two parameters, the beginning of track (BOT) address and an end of track (EOT) address. The addresses for BOT and EOT are stored in a
register file where they are easily changed. BOT and EOT addresses can be varied at any time under program control. With these programmable frame boundaries, the addressing of frames in the cache becomes dynamically variable.

A microprocessor in the cache manager stores a list of addresses for frames in the cache memory at which tracks of disk from different types of magnetic disk units may be stored. These addresses include a beginning of track address from which an
end of track address can be derived. These span a frame having enough memory locations to store an entire track for a particular type of disk unit. These addresses are supplied to a data and address assembler.

Records from a track of one of the disk units are stored in the frame specified by the beginning of track address and the end of track address. In accordance with the invention of copending Ser. No. 440,334 filed Nov. 9, 1982 and incorporated
herein by reference, the records are stored in the order received. That is, records are stored sequentially, but the first record is not necessarily stored immediately after the EOT address. Upon thereafter reading these records, it is desired to read
them out sequentially starting with the record at a specified starting address. In order to do this, beginning with the starting address each time a record is read, its address is incremented to form the next address. The incremented address is
compared to the end of track address. When the end of track address is reached, the next address is set to the beginning of track address. In this manner, records are read from the cache memory in a manner which emulates the reading of records from the
track of the disk unit.

It can be expected that at some point all the domains allocated to tracks of a particular length would be heavily in use, while those allocated to disk tracks of other type(s) were not. According to an important aspect of the present invention,
the domains are chosen to all be of the same size so that they can be efficiently reallocated to contain data stored on a different type of disk memory device. The domains are "dynamically" repartitioned into frames of a different size according to
usage. For this reason, the domain size is chosen such that the domains contain integral numbers of frames of various sizes. FIG. 2 shows an example of how this can be done. There the domain size is approximately 256 kilobytes (kb). In a preferred
embodiment, the cache contains some 12 of such domains for a total capacity of approximately 3 megabytes. Three possible domain subdivision schemes are shown in FIG. 2.

FIG. 2a shows a scheme which might be adopted for a domain to be used with a model 8650 disk drive sold by Storage Technology Corporation, the assignee of the present invention, in which each track contains some 19,069 bytes plus some identifying
"header" information. Thirteen such tracks with their headers will fit within the 256 kb domain. Storage Technology Corporation's Model 8375 disk drive has a track size of approximately 37,000 bytes. As shown in FIG. 2b, six such 37,000 kb tracks with
headers will fit within the 256 kb domain. Finally, Storage Technology Corporation's Model 8380 disk drive has a 47,476 byte track size. FIG. 2c shows that five such frames together with headers fit well into the 256 kb domain size.

According to the present invention, dynamic repartitioning of cache space allocation is provided in accordance with usage. This allows high storage efficiency without severe fragmentation. Fragmentation is wastage of space due to lack of
correspondence between the size of available frames and the sizes of the tracks that are to be stored in the available frames. Severe fragmentation is avoided because a domain may be reallocated from one device type to another by reframing whenever
desirable. Reallocation occurs in only one domain at a time, thus providing stability and simplicity by eliminating the need to invalidate the entire cache allocation scheme at anytime. A "domain use count" is kept to keep track of which class of
domains are experiencing heavy usage. This makes it possible to determine whether the current allocation of the domains is the most efficient with respect to usage of the system at any particular time. This is discussed below in connection with FIGS. 3
and 4.

A preliminary problem which must be addressed is frame assignment, that is, picking the best frame to assign to a given track to be staged to the cache array. The "best frame" is one which is not currently allocated to any track. However, if no
frames are free then a frame must be "deallocated" from the track to which it is currently assigned and reassigned to the new track which is to be staged. The best frame to deallocate is one which is no longer being read by the host. Frames which are
still being read by the host should not be deallocated because overhead spent in staging the track would not be recovered if it were deallocated before the data is read. See the copending application entitled "Detection of Sequential Data Stream",
referred to and incorporated by reference above for details of methods by which deallocation may be performed.

In a presently preferred embodiment of the invention, where no free frames are available frame assignment is performed using a "least recently used" (LRU) scheme. The LRU scheme is implemented by means of a list of frames, an example of which is
shown in FIGS. 5a-5c. The list has a top, at which is located the frames which have been most recently used, and a bottom, which corresponds to the frames which have not been used recently and those which are free. Thus, when a frame is used, that is,
is staged to or read from, its "name", i.e. its address and identifier, is placed atop the list. When a frame is deallocated, its name is moved to the bottom of the list. This occurs when the host directs a write operation to the area on disk from
which the contents of that frame were staged, thus rendering the data in that frame invalid, or when all records have been read from that track. Thus, the bottom of the list contains a free frame or the least recently used frame, and the frame on the
bottom of the list will typically be the best to assign to a track to be staged. Of course, when the frame is assigned, it is moved to the top of the list, indicating that it is the most recently used.

FIGS. 5a-5c depict the cache directory which is stored in the control store memory of cache manager 32. This directory contains the least recently used (LRU) list which is used to determine the domain and frame location addresses assigned to a
given track. The cache directory contains a linked list of frame locations and the status of each. FIG. 5a shows a series of frame descriptors 74, each describing a frame in the array. Each frame descriptor contains the status of its corresponding
frame. LRU (least recently used) pointers which maintain the linked lists are also shown. The frame descriptor 66 is the top of one LRU list, and stores addresses of the frames which have been most recently used; the descriptor 68 is at the bottom of
the list, and lists frames which have not been used recently and those which are free. Descriptors 66 and 68 are for one type of disk drive (type A). Descriptors 70 and 72 are similar, but are used for another type of disk drive (type B).

FIG. 5b exemplifies the ten 2-byte words 63 which make up each of the frame descriptors. The first two words (addresses 00 and 01) are used in searching for a particular track of data which has been stored in cache. The words stored at address
locations 02 and 03 are the LRU pointers, i.e., show where the corresponding frame is located on the LRU list. The pointer word at address 02 gives the address of the frame which is the next most active. The list has double pointers so the pointer at
address 03 specifies the neighbor frame toward the top of the list. The words at addresses 04 and 05 are the track addresses of the frame described by the descriptor. Address 06, byte 0 stores the frame status byte which is described more fully with
respect to FIG. 5c. The words stored at addresses 07 and 08 are the domain and cache addresses for the particular frame.

FIG. 5c shows in more detail the eight bits of the frame status byte, word 06, byte 0, of FIG. 5b. These bits are set to indicate the staging activity of the particular frame under consideration.

Bits 0-5 of the 8-bit frame status byte show specific flags indicative of various conditions relevant to the frame status. The six bits 0-5 are interpreted as follows:

Bit 0: Allocated/Free

1=The frame is allocated and contains valid data

0=The frame does not contain valid data

Bit 1: Device A/B

1=This is a device type `A` frame

0=This is a device type `B` frame

Bit 2: Defined Frame

0=This frame is undefined, that is, this slot for a frame in the directory does not define a frame.

1=This frame is defined

Bit 3: Stage In Progress

1=There is a stage in progress to this frame. The data is not yet valid but the frame is not free.

0=There is not a stage in progress to this frame.

Bit 4: Permanent Error

1=This frame contains a permanent error. The data is not valid, but the frame is not to be reallocated. The address of the error is stored where the track ID is normally stored.

0=The frame does not contain a permanent error.

Bit 5: In Use

1=This frame is currently in use. The device corresponding to this frame must not be selected.

0=This frame is not in use.

Thus, for example, if bit 4 of the frame status word is a "1", this indicates that this frame contains a permanent error (e.g., if one of the RAM chips within which the frame is stored is defective). Obviously it would be undesirable to assign
data to this frame for caching. Bits 1 and 2 are used in the reallocation process discussed below in connection with FIG. 4.

Thus, the cache manager contains a linked list of frame descriptors which specify the address and the status of each of a plurality of frames in the cache array 34. When writing to cache, in response to an input/output command, the domain
address and the beginning of track address of an available frame are obtained from the linked list of FIG. 5a. As noted, this is referred to as an "LRU" list (least recently used) because the top of the list contains frames which have been most recently
used and the bottom of the list contains frames which have not been used recently and are therefore available. In accordance with the invention of copending Ser. No. 440,334 filed Nov. 9, 1982, in the names of Swiatek et al, "Dynamic Addressing for
Variable Track Length Cache Manager," incorporated herein by reference, the LRU list provides a beginning-of-track (BOT) address of a frame in a domain assigned to a particular type of magnetic disk device. The number of bytes in a track of data for
this type of device is known, and it is added to the BOT address to obtain the end-of-track (EOT) address. In this manner, the EOT address is derived by the cache manager 24.

In writing a track to cache, the starting address is the first address after the BOT address which allows an adequate number of storage locations for the header. As a track is staged, or written, to the cache, the address location of each record
is stored in the header.

When reading from the cache, the starting address is the address of the record specified, as determined from the header.

Even given the LRU scheme, for assignment of specific frame addresses in domains to tracks to be cached, it may well be that if two or more classes of domains of different sizes are present in the cache memory, a first type will be much more
heavily used than a second type so that the LRU list for the first domain type will have, for example, fewer free frames on it than the list of the second domain type. In such cases, system performance can be improved by reallocating domains from the
second type of storage to the first. This domain reallocation method allows for efficient use of cache space. Stated differently, since the need for frames suited to a particular device will vary with time, it is desirable to dynamically allocate and
deallocate domains to device type based on demand. An important aspect of the present invention relates to a suitable dynamic domain allocation scheme; in a preferred embodiment, the invention supports two types of disk storage devices having differing
track storage capacities and can reallocate domains from, e.g., device type A to device type B, although it would be possible to provide three or more classes of domains in a given cache. In this way the cache space is allocated in proportion to the
input/output activity.

According to the presently preferred embodiment of the invention, three questions are considered by the domain allocation decision-making process:

How often to switch domains?

When to switch a domain?

Which domains to switch?

With respect to the first question, domain switching is considered in the presently preferred embodiment on the order of once every 1024 stage operations, i.e., every 1024 times a data track is read into the cache memory. In a relatively large
embodiment of the invention this is expected to occur approximately every 30 to 60 seconds.

The second problem is determining when to switch a domain. This decision should be based on an imbalance between the cache input/output activity ratios for device types A with respect to device type B. A suitable method of measuring activity is
to count the stages to domains assigned to each device type. This stage count ratio should be equal to the ideal "frames available" ratio. In turn, the frames available ratio can be used to compute an ideal "domains allocated" ratio since the number of
frames per domain is known. If the actual domains allocated ratio is different from the ideal domains allocated ratio it is appropriate to switch domains. In this way, the relative size of the various frames is removed from the consideration, as
opposed to merely measuring domain type activity.

The third and most difficult problem is to determine which domain to switch. Domains with many recently used frames are clearly not good candidates for switching. The best domains to switch are instead those that have many old and free frames.
Two alternatives may be considered. First, since the LRU list described above in connection with FIGS. 5a-5c shows the relative age of the frames of each domain, the list can be used to determine which domain to switch. A scan of a predetermined number
of frames on the list described above can be made, incrementing a counter for each domain when a frame belonging to the domain is found on the list. Accordingly, the domain having the lowest count at the end of the scan is that which is currently
undergoing the least activity. The scan should be limited to the top portion of the list, because this is where activity occurs. It might seem that the lower end of the list could be most profitably looked at because this is where the old and invalid
frames will congregate. However, if but a single active frame is in the same domain with a large number of old frames it would be invalidated as well by this kind of domain reallocation operation, which would be undesirable. The second alternative is
to count the stages to each domain rather than to examine the least recently used frame list. This would again provide an indication of relative activity.

Thus, it will be clear that the domain switching scheme according to the invention has two portions: the decision-making portion, which decides whether or not it is desirable to switch domains from one class to another, and an actual switching
process, in which it is determined which domain is to be switched. In a particularly preferred embodiment, the decision making process is table driven in order to save execution time; that is to say, when the device is turned on, a look-up table
indicating the ideal allocations of the domains for varying relative activity levels is generated and stored so that each subsequent reallocation decision can then simply be a look-up process not requiring an elaborate computation. Such a domain switch
decision table, usable in a preferred embodiment of the invention where the domain switching occurs between two device types, is shown in FIG. 3.

The table is built by making all the possible domain switching decisions at post-IMPL time (that is, upon initial microcode loading of the device) thus saving computation time when the decision is actually made. Making the decision to reallocate
a domain thus becomes a simple table look-up based on, e.g., the number of domains allocated to device type A and "A domain use"--a number representing actual staging operations from devices of type A--at a particular time. In the embodiment of FIG. 3,
the table shows numbers between zero and 15 (i.e., F, in hexadecimal) along the horizontal axis indicative of the relative number of stages to A device domains. While the vertical axis represents the number of domains allocated to A-type devices, each
column thus represents the A-use figure, while each row represents the number of domains allocated to device A at a given time. For each row, that is, for each number of domains allocated to A-type devices, there is an ideal target A-use. The table
thus provides a correlation between the number of A stages made in a given time and the number of domains allocated to A-type devices. If the actual figure is not equal to the ideal figure exemplified by the table of FIG. 3, then it is evident that
domain switching should be performed.

Target use is calculated based on the assumption that the ideal cache partitioning is achieved when: ##EQU1## One can usefully define a term called "Target A Stages", this being the number of stages which would occur to domains assigned to device
type A if the staging activity was in balance with cache allocation. This will lead to a target A frame use which can be compared with the actual A frame use to determine how close the actual stage activity compares with what will be ideal. Thus, in
the table of FIG. 3, each of the horizontal rows represents a given number of domains allocated to a device. The target A-use is then identical for that row. One of three indications is then filled in for each position of that row according to actual
use. A small a, indicating a B to A domain reallocation is "pending", i.e., desirable, is filled in at the places corresponding to actual A use being greater than target A use, thus indicating that switching of a frame from B to A is an acceptable
action if a request for an A frame assignment cannot be satisfied based on current allocation. Similarly, a small b indicating a "pending" switch from A to B is filled in at the positions corresponding to A-use being less than target A-use. Otherwise
an n, indicating a "no switching required" condition is filled in. Comparison of the actual use with the ideal relationship shown in FIG. 3 thus shows what reallocation should be performed, if any.

As will be observed from FIG. 3, several, typically 3, of the columns for each row have n's which indicate that no switch is pending. This is desirable in order to prevent unnecessary domain switching due to statistical variation in the number
of stages per domain type over a short period of time which can be expected to occur, while not indicating that domain reallocation is called for. The plurality of n's in the columns thus provide a quasi-damping effect on the domain switching performed
according to the invention.

Once it has been determined that a domain switch is pending, as above, if data is to be cached and no frame for the proper device type is free, and a domain switch operation is pending which would provide a frame of the proper device type, the
domain switching operation is initiated.

The first operation performed in the domain switching operation is to decide which domain would be the best to switch. The least recently used list of FIGS. 5a-5c of the device type to be switched is scanned to determine which domain has the
fewest frames on the more active portion of the least recently used list. If this domain in turn has no frames marked "In Use" or "Stage In Progress", it is presumed to be the best one to switch. Otherwise, the domain with the next fewest frames in the
top of the least recently used list is chosen. Once a domain is chosen for switching, the domain is converted to the new device type. All valid frames belonging to the domain are deallocated. All frames in the domain are removed from the least
recently used list of the device type from which the domain was switched. For example, if a type A domain is thus deallocated, all frames in the domain are removed from LRU descriptor 68. The number of new frames that will fit into the new domain are
linked onto the bottom of the new device type's least recently used list. In the example, they are written to the descriptor 72. Finally, new pointers to the array are assigned for each frame according to the frame sze of the new device type. Thus,
when the domain conversion routine is finished, the new frames are at the bottom of the new device type's least recently used frame list and are marked free.

FIG. 4 shows a schematic view of the decision-making process according to the invention. As discussed above, at initial program load, "IMPL time", at 30, the table is written with all the possible decisions appearing as at 32, the table then
appearing as in FIG. 3. This information is then available to domain switch determination module 34. This decision making process is also supplied with domain switch statistics as at 36 which indicate whether domain usage is presently corresponding to
the domain allocation. Therefore, when a stage request is validated as at 38 by the cache controller 24, (FIG. 1) as discussed in copending application, Ser. No. 325,350 filed Nov. 27, 1981 for "Detection of Sequential Data Stream" and a
continuation-in-part of that application, having the same title, Ser. No. 441,901 filed Nov. 15, 1982, the domain switch statistics may be examined to determine which domain, if any, should be switched. Typically, the operation is performed every 1024
stages. If an imbalance is detected by accessing the table of FIG. 3, as at 40, a flag is set indicating that a switch is pending. Thereafter, upon a request that a frame be allocated to a data record, if no free frame is available, and if an
appropriate switch is pending, the least recently used domain is switched as described above as indicated at 44. The domain thus converted at 46 has now been allocated to the new device type, e.g., type A, and the LRU tables are updated to reflect this
fact at 48, while the directory of frames available is likewise updated as at 50. Similarly, the frame status byte of each frame (FIG. 5c) are updated; bit 1, defining the device type to which the frame is assigned, would be switched, and bit 2 would be
set to 1.

Attached hereto as Appendix A and incorporated herein by reference is a document entitled "Module: Domain Switching" which is a detailed description of methods used to implement the invention. This document is largely written in a form of
convenient shorthand language referred to as pseudocode, the grammar of which is explained in a document also attached hereto and incorporated by reference marked Appendix A-1, entitled "Chinook Prolog and Pseudo-Code Conventions." Those skilled in the
art supplied with the domain switching document of Appendix A, together with the interpretative pseudocode convention document would have no difficulty implementing the methods of the invention.

Those skilled in the art will recognize that there has been described a domain partitioning system for dividing up a solid-state memory which fulfills the needs of the art and objects of the invention mentioned above. Specifically, use of
domains having a size into which commonly accessed quantities of data, e.g., tracks, fit conveniently provides efficient storage of data without fragmentation as old records are overwritten by new data. More particularly, the provision of an adaptive
domain switching scheme allows efficient utilization of memory to be achieved even when relative domain utilization changes from time to time. Furthermore, it will be appreciated that the metod of the invention has applicability beyond the two-device
embodiment specifically described, and that therefore the description of the invention given above should not be considered as a limitation on its scope but only as exemplary thereof. The scope of the invention is more properly limited by the following
claims. ##SPC1## ##SPC2##